1. Field of the Invention
The present invention relates to a process and detector for the determination of chemical compounds by chemiluminesence with ozone.
Essentially the process of the present invention consists in mixing vapors of chemical substances with a flow of ozone in a special reaction chamber or, more specifically, in a detector in which these substances are subjected to the ozone attack. From this reaction results the emission of luminous radiation, called chemiluminescene, which may or may not be transmitted through an optical system and is directed to a photomultiplier tube. The current generated in the photomultiplier tube is amplified in an amplifier and recorded in any convenient graphic system. In this way qualitative and quantitative determinations can be made of organic and inorganic substances capable of emitting chemiluminescence upon reacting with ozone.
When a gaseous mixture is passed through a chromatographic column, and the components thus separated are introduced into the reaction chamber, according to the present invention, and mixed with ozone so that a reaction and emission of luminous radiation will take place, the reaction chamber is embodied by a detector for gas chromotography determinations.
2. Description of the Prior Art
Numerous detectors of chemical substances are already being used in gas chromatography. The most well-known are the following:
I--Thermal detectors: detector of thermal conductivity; thermal absorption detector; flame detectors, such as hydrogen flame temperature detectors, of flame emmissivity detectors, or hydrogen flame ionization detectors, of flame photometric detectors, of alkaline flame ionization detectors.
II--Detectors of non-radioactive ionization: thermoionic detector, photoionization detector, gas discharge detectors.
III--Detectors of radioactive ionization sources: detector of square section, argon detector, detector of electronic mobility, detector of electronic capture.
IV--Titrimetric or coulometric detectors.
V--Various detectors, results of corresponding adaptations of radiation counters, gravimetric, potentiometric and voltametric devices, analytical instruments, such as mass spectrometers, infrared or ultraviolet spectrophotometers, atomic absorption spectrometers, and the like.
Among the detectors used in gas chromatography the most common ones are those of thermal conductivity, of hydrogen flame ionization, electronic capture, alkaline flame ionization, and flame photometry.
The detector of thermal conductivity using thermistors or a heated filament and also called katharometer is one of the oldest and still the most widely used. With this detector the thermal conductivity of the gaseous mixture entering a sensitive cell is compared with the thermal conductivity of the pure carrier gas which flows constantly through the reference cell. It is a differential detector sensing the concentration of the substances present in the carrier gas, and a universal detector capable of responding to any vaporizable substance other than that used as the carrier gas.
In the hydrogen flame ionization detector the substances issuing from the chromatographic column are mixed with a hydrogen stream and burnt in a small burner, in the presence of air, producing a flame which is invisible to the naked eye. The temperature of the hydrogen flame is sufficient to ionize an organic substance, the ions of which when collected on electrodes, generate an electric current which is then amplified and transmitted to the graphic recorder. This is a specific detector for organic substances, insensitive to inorganic substances, such as fixed gases and water. The detector responds to the total mass of the burnt substance per time unit.
The detector of electronic capture employs a radioactive source to generate the ions to be measured by the detector. The radioactive source produces free electrons of great velocity which are captured by the molecules of the carrier gas used, forming negative stable ions or charged atoms, and thus generating a current. The capability of capturing free electrons is dependent on the electronic affinity of each molecule. If an organic substance separated in a chromatographic column enters the chamber of the detector and if this substance has greater electronic affinity than that of the carrier gas, some of the electrons will be captured and the initial current reduced in dependence on the quantity of the substance present and on its electron affinity, generating a signal which is recorded. This is an extremely sensitive detector. However, only substances having this property of electron capture can be detected, such as halogen containing compounds, nitrates, conjugated carbonyls and some organometallic compounds.
The alkaline flame detector is similar in operation and structure to the hydrogen flame ionization detector, the difference residing in the fact that a small quantity of an alkali salt, normally cesium bromide or rubidium sulphate, is placed close to the flame. The salt may be in the form of a tablet or it may cover a fabric or threads of various geometric configurations. When the hydrogen flame of the substance to be analysed burns in the presence of a weak current of air near an alkali salt, a current is generated which is about 100 times greater than that of the common hydrogen flame. This current increases considerably when the substance to be burnt contains phosphorus or halogen. Although the theoretic basis of the functioning of such a detector is still unknown, this is a device widely used in the determination of organophosphorus compounds in pesticides and polluents. It is a detector of great specific sensitivity for compounds containing phosphorus and for some compounds containing halogen.
The flame photometry detector is based on the fact that a hydrogen flame emits light in the presence of air. The carrier gas transporting a substance separated in the chromatographic column is mixed with an oxygen enriched air stream, passed on to a burner, and at the exit thereof hydrogen is added. The resulting gas mixture contains hydrogen in excess for complete combustion with the oxygen present. The luminous radiation caused by this combustion impinges upon a mirror and is reflected on to an optical filter which selects the desired wavelength (526 millimicrons for phosphorus-containing substances and 394 millimicrons for sulphur-containing substances). Subsequently it passes to a photomultiplier tube, the current of which is amplified and recorded. This is a specific selective detector for sulphur-and/or phosphorus containing substances since the observed emittance results from the formation of molecular specimens of S.sub.2 and HPO during the burning in the hydrogen flame. It is a highly sensitive detector capable of detecting nanograms and is much used in pollution control and determination. It may be constructed as a channel so that only one type of substance is determined, according to the filter chosen. It may also comprise two channels, each furnished with an appropriate filter so as to detect sulphur- and phosphorus-containing compounds at the same time.
The functioning principle of the detector according to the invention is entirely different from that of the detectors described above, especially from that of the flame photometry detector because, although luminous emission does occur, there is no combustion, a fact requiring peculiar characteristics of the reaction chamber, as will become apparent from the specification below.
Chemiluminescence was already made use of in pollution control to determine ozone in the atmosphere. In this case the stream containing ozone is mixed with an ethene flow to provoke a reaction and consequently the generation of chemiluminesence, for instance as described by W. A. Kummer, J. N. Pitts, Jr. and R. P. Steer in the article "Chemiluminescent Reactions of Ozone with Olefins and Sulfides" in Environmental Science & Technology, 5 (1): 1045-1047, (1971). Also, the ozone may be applied to act on a coloring substance, for instance Rodamine B as cited by W. R. Seitz and M. P. Neary in an article entitled "Chemiluminescence and Bioluminescence in Chemical Analysis" published in Analytical Chemistry, 46/2/188A-202A, (1974).
Also chemiluminescence by reaction with ozone was already used in the determination of nitrogen oxides, NO and NO.sub.2, for control of atmospheric pollution. In this case an atmosphere sample is caused to react with ozone in a chamber equipped with a photometric detector. Chemiluminescence results from the reaction of the ozone with NO in the wavelength range of 0.6 to 3.0 microns. Normally the devices used comprise a reduction zone containing activated carbon so as to convert NO.sub.2 into NO. They determine the total of nitrogen oxides. The NO.sub.2 concentration then is the difference between the oxide total and the NO concentration.
In the above cases the chemiluminescence produced by the reaction of ozone was used only in very specific instances in which the substance to be determined was the ozone proper or an inorganic gas such as NO.